An overview of hand anatomy, including:
- Bones of the hand
- Muscles of the hand
- Blood supply of the hand
- Innervation of the hand
The bones of the hand
Proximal to the hand are the 8 carpal bones.
The radius articulates with the cashew shaped scaphoid bone laterally and the croissant shaped lunate.
The ulna articulates with the pyramid shaped triquetrum via a pad of triangular fibrocartilage.
The trapezium articulates with the first metacarpal (hence ‘trapezium with the thumb’), next to which lies the trapezoid.
The capitate connects to the middle metacarpal and the hamate (which has a hook on its palmar surface) articulates with the remaining two metacarpals on the ulnar side.
The pisiform is a pea shaped sesamoid bone that sits on the triquetrum and lies within the tendon of flexor carpi ulnaris.
The metacarpals articulate with the proximal phalanges, which articulate with the middle phalanges, which finally articulate with the distal phalanges.
The thumb has only a proximal and distal phalanx. It opposes the tips of the other fingers and is essential for precision grip.
The interossei muscles originate between the metacarpals.
There are 4 dorsal and 3 palmar interossei.
They insert onto the proximal phalanx and extensor hood of each finger.
Palmar interossei adduct the fingers, and dorsal interossei abduct the fingers (hence PAD / DAB).
The radial artery enters the hand by passing between the two heads of the first dorsal interosseus.
The 4 lumbricals are thin worm like muscles that flex the metacarpophalangeal joints and extend the interphalangeal joints.
They arise from the tendons of flexor digitorum profundus.
Thenar & Hypothenar eminence
Thenar (thumb side) eminence:
- Opponens pollicis is deep
- Flexor pollicis brevis is on the ulnar side of the eminence
- Abductor pollicis brevis is on the radial side of the eminence
Hypothenar (little finger side) eminence:
- Opponens digiti minimi is deep
- Flexor digiti minimi is on the radial side of the eminence
- Abductor digiti minimi is on the ulnar side
The ulnar nerve (C8-T1) supplies all the intrinsic muscles, apart from the muscles of the thenar eminence and the radial two lumbricals. These muscles are supplied by the median nerve (C5-T1).
The median nerve supplies sensation to the radial 3 and a half fingers on the palmar aspect as well as the nail beds.
Sensation to the palmar and dorsal side of the ulnar one and half fingers is supplied by the ulnar nerve.
The radial nerve supplies the radial 3 and a half fingers on the dorsal side.
Blood supply and drainage
The superficial palmar arch is the main continuation of the ulnar artery. It receives a small superficial branch from the radial artery and supplies the fingers with blood via the proper digital arteries.
The deep palmar arch is the main branch of the radial artery, and supplies the deep hand structures.
Paired veins accompany the arterial arches and share the same names i.e. radial and ulnar. The more superficial cephalic and basilic veins drain the dorsal venous network of the hand.
Carpal tunnel syndrome
If the median nerve becomes compressed within the carpal tunnel, there is paraesthesia in the radial 3 and half fingers as well as thenar muscle wasting.
Usually, the more proximal a nerve injury, the worse it is. The opposite is true when we consider the ulnar nerve. This is because one of the muscles that flexes the fingers (Flexor digitorum profundus, which lies in the forearm) is partially innervated by it. Hence a proximal injury will remove innervation to the forearm muscles and the hand muscles. A distal injury only takes out the hand muscles; hence the still functioning finger flexors give the patient a clawed appearance in the ring and little finger. With a proximal injury leading to an open palm, there is more capacity for hand function.
The palmar aponeurosis is a thick area of fascia that is tightly attached to the skin. It may thicken and contract which causes the little and ring finger to flex abnormally.
The scaphoid receives its blood supply from a nutrient branch of the radial artery. It enters at the distal pole of the bone, and runs to its proximal part. A fracture of the scaphoid (which can result from a fall onto an outstretched hand) can therefore lead to avascular necrosis of the proximal bony fragment. Symptoms include a tender anatomical snuffbox.